Electronic devices formed from a gallium nitride layer epitaxially grown on a high resistivity silicon substrate naturally form a high conductivity layer at the gallium nitride/silicon interface. As the gallium nitride or an aluminum nitride layer is epitaxially grown, high temperatures are applied to the silicon. As a result, the epitaxially grown layer can diffuse into the silicon and thereby dope the silicon causing the resistivity of the silicon to decrease. For example, as shown in FIG. 1 during growth of a AlGaN supperlattice or graded buffer on a silicon substrate, the Al and Ga can act as p-type dopants forming a doped Si region.1 The decrease in the resistivity is caused by the appearance of free charge at the boundary between the two materials. Those skilled in the art often refer to this high conductivity layer as an “inversion layer.” The inversion layer which acts as a conductor, undesirably electromagnetically couples with circuits of the gallium nitride layer, effectively forming a significant source of power loss and noise in the system. 1 Pattison et al, Improving GaN on Si Power Amplifiers through reduction of parasitic conduction layer, Proceeding of the 9th European Microwave Integrated Circuits Conference (Oct. 7, 2014)
The parasitic conduction layer is represented by a resistance termed Rbuffer and the value of Rbuffer being dependant on the depth and doping concentration of the Si substrate. Additionally, capacitive coupling can be seen between the GaN, buffer, and Si layers.
The RF performance of such an electronic device consequently often is determined by the nature of the inversion layer. Specifically, the inversion layer can effectively short circuit at certain RF frequencies by capacitively coupling to circuits in the gallium nitride layer.